Turbine blades in modern gas turbine engines can be exposed to high operational temperatures, such as temperatures in the high-pressure part of the turbine. For this reason, such turbine blades can be provided with internal passages through which cooling air is circulated. Cooling air which is bled from one or more compressor stages in the gas turbine engine, can impose a performance penalty on the engine. In such a case, the blade designer can seek to minimise cooling air consumption by designing the blades with complicated internal cooling passages. Modern high pressure turbine blades can be manufactured using the “lost wax” shell moulding process, in which the internal cooling passages are defined within the wax blade shape by cores made of a ceramic or other leachable material. When the wax is melted out of the shell mould and replaced by molten metal alloy, the ceramic cores remain in the solidified cast blade to define the internal cooling passages. The ceramic cores are removed during the last stages of the manufacturing process by, for example, a leaching process that dissolves the ceramic cores out of the blade internals using a caustic chemical composition.
FIG. 1A shows a longitudinal (root to tip) section through a known high pressure turbine blade 10, in which the arrows show the directions of the air cooling flows. An internal cooling passage 12 follows an “up-and-down” route through the blade, in which a first leg 12a of the passage extends from an inlet 14 at the root of the blade up to the blade tip, a second leg 12b doubles back on the first leg 12a, and a third leg 12c doubles back on the second leg 12b, before the passage terminates at a dust hole 16 in the blade tip. In this way, an increase in cooling duty can be obtained from the cooling air. Because passage 12 was defined in the casting by a ceramic core or the like, dissolving the core from the parts of passage 12 that are remote from the inlet 14, such as from the bend zone 18 between legs 12b and 12c, can be particularly difficult. Leaching out the ceramic core in this zone can take a long time, thereby adding expense to the manufacturing process, and unless particular care is taken, there is a possibility that remnants of the core will remain inside the cooling passage.
It is known from EP-A-1 267 040 and other documents to define small openings in internal cooling passage walls of the casting by thin ancillary core portions that join one part of the ceramic core to another part. This can be done to provide support to cores during the casting process. After the part is cast and the core has been leached out, the opening can be closed off with a plug that is securely fixed into place.